The present invention relates to a process for synthesizing a polysilazane from a halosilane adduct, which may or may not be isolated, to the halosilane adduct, to the polysilazane, and to a method of preparing silicon nitride from the polysilazane.
Sintered silicon nitrides are important high-temperature structural materials for gas turbines and diesel engines because of their excellent high-temperature strength, thermal shock resistance and oxidation resistance. They are also important as high-performance materials for cutting tools, etc., since they contribute to the saving of energy and resources.
The silicon nitrides have been produced by (1) a direct sintering process wherein metallic silicon powder is directly nitrided by heating to 1300.degree. to 1500.degree. C. in a nitrogen or ammonia stream, (2) a silica reduction process wherein silica or a silica-containing substance is reduced with carbon by heating it in a nitrogen atmosphere and silicon thus formed is reacted with nitrogen, (3) a gas-phase synthetic process wherein silicon tetrachloride is directly reacted with ammonia at a high temperature and (4) an imide thermal decomosition process wherein silicon diimide obtained by ammonolysis of silicon tetachloride is heated in a non-oxidizing atmosphere to obtain silicon nitride.
However, the above process (1) requires a long reaction time and complicated heating steps and the resulting silicon nitride comprises mainly rough, .beta.-type silicon nitride having a high impurity content. The process (2) has defects that the purification of the starting material is difficult, that a long reaction time is required and that the obtained product is a mixture of .alpha.-type and .beta.-type silicon nitrides. In the process (3), the resulting silicon nitride is generally amorphous. Though the process (4) has an advantage that .alpha.-type silicon nitride having a high purity can be produced in a high yield, it has also a disadvantage that, since silicon diimide [Si(NH).sub.2 ].sub.x as a precursor of silicon nitride is insoluble in a solvent, the use thereof is limited in practice.
Recently, a process has been proposed wherein a polysilazane obtained by thermally decomposing an organic polysilazane is heated to 800.degree. to 2,000.degree. C. to form silicon nitride by calcination [see Hajime Saito, "Sen'i Gakkai-shi" Vol. 38, No. 1, pp. 65-72 (1982)]. This process has, however, a defect that silicon carbide and free carbon are formed in addition to silicon nitride.
On the other hand, inorganic polysilazanes soluble in solvents were synthesized by A. Stock et al. in 1921 and it was proved by Seyferth et al. in 1982 that they were useful as precursors of silicon nitride.
However, conventional processes for producing inorganic polysilazanes wherein dichlorosilane having a high vaporizing is used have the following defects: (1) ammonium chloride obtained as the by product and the polysilazane are deposited on the walls of gas tubes and reactors in the reaction apparatus to clog the gas duct, (2) in order to prevent this trouble, the reaction temperature should be kept low to inhibit the scattering of dichlorosilane, and (3) dichlorosilane having a high toxicity and flammability should be kept in a closed vessel at a low temperature. The polysilazane synthesized by the above Stock's process is only an oligomer of the formula: (SiH.sub.2 NH).sub.n in which n is a number of 7 to 8 whichis in the form of a viscous liquid at ambient temperature. The product obtained by the process of D. Seyferth is in the form of an oily liquid having a proton ratio of Si--H/N--H of about 3.3. This product solidifies by heating to about 200.degree. C. or by being left to stand at room temperature for 3 to 5 days. The properties of all of these polysilazanes were insufficient for the precursors of silicon nitride shaped at ambient temperature.